Isohexide Derivatives from Renewable Resources as Chiral Building Blocks

The combination of rapidly depleting fossil resources and growing concerns about greenhouse gas emissions and global warming have stimulated extensive research on the use of biomass for energy, fuels, and chemicals.[1] Although biobased chemicals have the potential to reduce the amount of fossil feedstock consumed in the chemical industry today, the most abundant type of biobased feedstock, that is, carbohydrates, is often unsuitable for current high-temperature industrial chemical processes. Compared to hydrophobic aliphatic or aromatic feedstocks with a low degree of functionalization, carbohydrates such as polysaccharides are complex, overfunctionalized hydrophilic materials. One approach to overcome these drawbacks is to reduce the number of functional groups, resulting in more stable, industrially applicable bifunctional biobased building blocks,[2] such as furan-2,5-dicarboxylic acid,[3] levulinic acid,[4] and isosorbide.[5] Isosorbide (1,4:3,6-dianhydrosorbitol) is a rigid bicyclic diol that is derived from sorbitol and can ultimately be obtained from glucose-based polysaccharides such as starch and cellulose.[6] Apart from isosorbide, in which the hydroxyl groups on C2 and C5 are in the exo and endo positions, respectively, two other isohexides are known (Figure 1): the symmetrical endo-endo isomer isomannide (1,4:3,6-dianhydromannitol) and the exo-exo isomer isoidide (1,4:3,6-dianhydroiditol), derived from d-mannitol and l-iditol, respectively.